Biaxially oriented heat-shrinkable films of the monolayer type are used in the packaging of poultry. Biaxially oriented heat-shrinkable films of the multilayer type having an oxygen barrier core layer are used in the packaging of processed meat and fresh red meat. As generally understood, a "heat-shrinkable" film tends to return to its original unstretched (unextended) dimension when heated to its softening point. The terms "orientation" or "oriented" are used to describe the manufacture of heat-shrinkable films, wherein resin material is heated above its flow or melting point and extruded through a die into either tubular or sheet form. After cooling, the relatively thick extrudate is reheated to a temperature range suitable to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials is understood by those skilled in the art to be in a range which revises the inter-molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as shrink. When the stretching force is applied in one direction, uniaxial orientation results. When the stretching force is simultaneously applied in two directions, biaxial orientation results.
A commercially used thermoplastic material in the manufacture of biaxially oriented heat-shrinkable films is ethylene vinyl acetate (EVA). This material in the thin film form (e.g. 2.5 mils) is characterized by high shrink properties, for example at least 35% shrink in both the machine (MD) and transverse directions (TD).
Biaxially oriented heat-shrinkable films containing EVA are commonly produced by extruding a primary tube, cooling, and then reheating and expanding the primary tube both longitudinally and transversely by means of different nip roll speeds in the longitudinal direction and air inflation in the transverse direction. One such process is described in the U.S. Pat. No, 3,456,044. This two-step process is often referred to as the double-bubble or the trapped bubble process. When a multilayer film is desired the multiple layers may be coextruded, as for example with an oxygen-barrier core layer and first and second outer layers on each side of the core layer to form the aforementioned primary tube. As for example described in Canadian Patent No. 982,923, these outer layers may comprise EVA and the core layer may comprise a vinylidene chloride copolymer with a comonomer such as vinyl chloride or methyl acrylate. Another commonly used oxygen barrier material is hydrolyzed ethylene vinyl alcohol, i.e. EVOH. Instead of coextrusion the primary tube may be formed by coating lamination, wherein a first outer layer is extruded and thereafter the core and second outer tubular layers are sequentially coated onto the outer surfaces of the first tubular layer and the core layer. As another alternative, the first outer and core outer layers may themselves be coextruded, and the second outer layer thereafter coated onto the outside surface of the core layer. Coating lamination procedures are described in U.S. Pat. No. 3,741,253. As still another alternative, a multiple layer film may be formed as a sheet by the well-known slot casting procedure, and then biaxially oriented into a heat-shrinkable film.
One limitation of EVA-based biaxially oriented heat-shrinkable film is that the plastic orientation strength of EVA films is relatively low compared with certain other thermoplastic materials as for example polyethylene. Because of its molecular structure, EVA has relatively low strength in the primary tube form when reheated to orientation temperatures as for example 68.degree.-84.degree. C. As a result, EVA based biaxially oriented heat-shrinkable film is subject to bubble breaks and process interruptions, which can result in a relatively high waste rate.
Certain polyethylenes may be used to manufacture biaxially oriented heat-shrinkable films, and their plastic orientation strength is substantially higher than EVA. Unfortunately their heat shrink properties are inferior to EVA. By far the best heat shrink properties in the polyethylene family are achieved with very low density polyethylene (VLDPE). Although the heat shrink properties of VLDPE approach those of EVA, there is a significant difference and loss of shrink if VLDPE is substituted for EVA in the production of biaxially oriented heat-shrinkable films to achieve higher plastic orientation strength. Moreover it has been determined that the shrink and plastic orientation properties of EVA and VLDPE blends are approximately linear. That is, the shrink and plastic orientation properties of biaxially oriented heat-shrinkable films comprising 50% EVA - 50% VLDPE are approximately mid-way between those of the pure components.
An object of this invention is to provide an improved biaxially oriented heat-shrinkable film containing EVA and VLDPE, which has machine direction (MD) and transverse direction (TD) shrink properties similar to those of a pure EVA film.
Another object is to provide such a film having plastic orientation properties similar to those of a pure VLDPE-film.
Still another object is to provide a biaxially oriented heat-shrinkable film containing EVA and VLDPE as principal components, having the high shrink properties of a pure EVA film but also the improved plastic orientation strength properties of a pure VLDPE film.
A further object is to provide an improved method for manufacturing a biaxially oriented heat-shrinkable film containing EVA and VLDPE wherein the biaxial orientation is achieved by bubble expansion, but wherein the strength of the bubble is similar to that of a pure VLDPE bubble, and wherein the method imparts biaxial heat shrink properties similar to those of EVA.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.
As will be explained and demonstrated hereinafter in more detail, the biaxially oriented heat-shrinkable film of the present invention overcomes the aforedescribed limitations of EVA and VLDPE heat-shrinkable films and provides the most desired properties of each in a single film, i.e. the plastic orientation strength of VLDPE and the high MD and TD shrink properties of EVA. At the same time the instant film has physical properties which on balance are at least as favorable as presently used films employing EVA/VLDPE blends. This comparison has been made between both nonirradiated and irradiated films.
As used herein, "heat-shrinkable" means that the film has at least 10% free shrink measured at 90.degree. C. in at least one direction, i.e. machine or transverse, in accordance with ASTM D-2732.